This invention relates to plasma torches such as for heating a gas and particularly to plasma torch electrodes, their composition, and methods of manufacture.
Plasma torches to be improved by the present invention typically contain two tubular shaped, water cooled, electrodes colinearly arranged along an axis. In direct current operation, one electrode is at a high potential and the other is normally at ground potential. There is a small gap, typically about 1 mm, between adjacent ends of the electrodes where an arc is initiated during startup. Gas to be heated is forced through this small gap into the inside of the tubular electrodes, thereby causing the arc to be extended into their inside diameter. Field coils surrounding the electrodes cause the arc to rotate within the electrode bores at a high velocity. The cold gas, coming through the small gap and then through the rapidly moving arc, is thus heated by the arc.
One electrode is referred to as the upstream electrode and normally has a closed end and is normally the electrode to which a high potential is applied. The other electrode, at ground potential, has an open end from which the heated gas passes and is referred to as the downstream electrode. The heated gas may be utilized for any number of heating purposes including chemical processes such as ore reduction.
Further background on relevant torches may be found in U.S. Pat. Nos. 3,705,975 and 4,214,736, whose description as to the construction and manner of operation of plasma torches is incorporated herein by reference.
Electrode life, particularly at the upstream, high voltage electrode, is a concern with the foregoing and similar torch designs, particularly when operating with an oxidizing gas such as air as the torch gas with copper electrodes.
As a consequence, a limited life of the electrodes for a given power level and torch size has limited the use of torches in commercial applications.
Another important factor is that in most industrial torch applications, the replacement of worn electrodes results in significant lost operating time for the process. Hence, longer lasting electrodes are desirable even with somewhat added cost for such electrodes.
During normal direct current operation on copper electrodes with the upstream electrode being the anode, the life of the upstream electrode may be less than about 100 hours and the life of the downstream electrode may be less than 300 hours. Oxide particles coming from the upstream electrodes tend to cause unstable torch operation. Copper oxide is stable at high temperature. These small particles enter the gap between electrodes, causing periodic short circuits and damage to the gap area. Reversing the polarity does not avoid the problem. Torch operation on alternating current alleviates the gap shorting problem somewhat but the electrode life of the two electrodes is merely made substantially equal at about 200 hours or less.
While copper has been the commonly used electrode material (typically OFHC copper with purity greater than 99%), exhibiting the above-mentioned wear problems, some longer life torch electrodes have been made of silver and copper alloys in the range of 72% to 90% silver. While the use of electrodes of such a composition has been found favorable in terms of lifetime when operating on air or oxygen, the expense of the electrodes has prohibited very widespread use. The relatively high cost results both from the cost of the silver electrode material itself as well as from the required fabrication operations.
Some electrodes in small torches made by Westinghouse have consisted entirely of a silver-copper alloy of the eutectic composition of 72% silver-28% copper. The electrodes were made by extruding the material from a rod. In other work reported by C. B. Holden of PPG Industries, Inc. in a paper "Electrode Life in An Arc Heater" (publication citation not known), life problems of electrodes are reported and discussion of the characteristics of silver alloy electrodes is given. The 72%-28% silver-copper alloy was recommended; certain commercial arc heater electrodes were made of the 80%-20% silver-copper alloy. Both the anode and cathode had a copper ring brazed onto one end to permit a threaded connection. Also, it is reported that a step joint and silver solder were used to fit deteriorated electrodes with new noses to replace the damaged area of the same 80%-20% alloy. In the case of some rear electrodes, this joint technique is also reported to have been used using a length of silver alloy tubing where the arc attachment was expected and copper tubing at both ends. The silver alloy tubing used for these electrodes was of cast material. It is mentioned that at the end of their lifetime of 5,000-10,000 hours (with an arc drawing about 550 amperes), they could be repaired with a new section of silver alloy tubing replacing the eroded part, giving even greater length of useful life.
The foregoing results in considerable material cost and, also, concern about the integrity of soldered joints which are required to be water tight. In some torches of particular current interest, the current drawn is in the range from about 1000-2000 amperes which aggravates the problem of electrode life.
In general, silver electrode material is typically more expensive than copper by a factor of about 30. Further, the fabrication of silver into the shape required for manufacturing electrodes might double this unfavorable ratio. Actual test data measuring wear on an anode indicates electrode life extended by factors of about 7 to 10 times in the high wear region of the electrode surface when using silver alloy material as compared to copper. An objective of the present invention is to provide designs for electrodes and their fabrication that are sufficiently economical so that the cost disadvantage does not greatly offset the improvement in life time.
In accordance with the present invention, a torch electrode comprises a tubular outer shell of a first material such as copper. On the inner surface of the outer shell, or preferably merely a portion of the inner surface, is directly fabricated an arcing portion of a second, more durable, metal such as silver-copper alloy. The second metal is provided, at least, in the region where the arc normally attaches to the electrode surface under the operating conditions to be encountered. In one method a silver alloy powder is compacted onto the shell by a hot isostatic pressing process. In another method, the silver alloy in the form of a powder or other form such as a wire can be placed in a cavity between the shell and a liner and then melted in a furnace to form a cast layer of alloy in the proper location. By such techniques, the occurrence of the silver alloy can be minimized both in axial extent as well as in thickness. A silver alloy thickness of no greater than about 6 mm, on the copper outer shell, is sufficient to provide a lifetime extension of about 7-10 times as compared to copper with an economical cost. The silver alloy thickness is generally no more than about half of the total electrode thickness. This is to extend life with lower material cost. A complete electrode, or complete thickness of silver alloy provides only a marginally greater improvement in life but at a considerably greater cost. While significant advantage can be taken of such electrodes as provided in a unitary integral structure, it is also a suitable design to provide the copper shell in detachable sections, as by having threaded ends, with the use of O-ring seals as desired, in order to permit replacement of only a section of the shell when the section having the arcing portion becomes worn.